1. Field of the invention
The present invention relates to a light modulator and, more particularly, to a modulator using a semiconductor laser diode.
2. Description of the Prior Art
It is known in the art to use semiconductor diodes for transmission systems, such as video signal transmission systems. Such a transmission system is generally referred to as an optical transmission system. Although the optical transmission system has advantages in wide modulation bandwidth and high coupling efficiency, it also has the following problems.
First, speckles in optical fibers, generated by the interference among propagating modes in the optical fibers due to the change in the coupling coefficient at junctions of optical fibers, adversely affect the linearity of the signal propagation, resulting in the generation of noise signals which are generally called speckle noise.
Another problem is the laser diode linearity deterioration caused by the light reflected back to the laser from the optical fiber at the coupler, resulting in the generation of unwanted reflection noise signals.
To reduce such noise signals, the signal to be transmitted is superimposed by a high frequency carrier (about 1 GHz). When such a high frequency signal is applied to a semiconductor laser diode, the laser diode performs a multilongitudinal mode oscillation, thereby widening the oscillation spectral width. This results in chirping, i.e., the undesirable variation of wavelength of the laser generated from the laser diode, and also results in the reduction of the coherence of the laser. This is further disclosed in an article "Speckle Noise Reduction in Fiber Optic Analog Video Transmission Using Semiconductor Laser Diodes" by Sato et al presented in IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. COM-29, No. 7. July 1981, pages 1017-1024.
An example of a prior art light modulator is shown in FIG. 1 in which a data signal generator 2 generates a data signal to be modulated, such as a video signal. The data signal from generator 2 is superimposed on a high frequency carrier signal provided from high frequency signal generator 3 through capacitor 4. The superimposed signal is applied to semiconductor laser diode 1 at which the light modulation is carried out.
The prior art light modulator has the following disadvantages. Since the semiconductor laser diode has a relatively large value of coupling capacitance, the total capacitance of the light modulator amouonts to a large value, resulting in high power of high frequency signal on which the data signal is superimposed. Therefore, it is necessary to prepare a high frequency signal generator 3 which is capable of producing a high power output, resulting in a high manufacturing cost. Also, such a high power output accompanies unwanted electromagnetic wave emission which enhances the noise signal.